compiler.py 文件源码

def prepare_value(self, field, value):
        """
        Prepare a value to be used in a query by resolving it if it is an
        expression and otherwise calling the field's get_db_prep_save().
        """
        if hasattr(value, 'resolve_expression'):
            value = value.resolve_expression(self.query, allow_joins=False, for_save=True)
            # Don't allow values containing Col expressions. They refer to
            # existing columns on a row, but in the case of insert the row
            # doesn't exist yet.
            if value.contains_column_references:
                raise ValueError(
                    'Failed to insert expression "%s" on %s. F() expressions '
                    'can only be used to update, not to insert.' % (value, field)
                )
            if value.contains_aggregate:
                raise FieldError("Aggregate functions are not allowed in this query")
        else:
            value = field.get_db_prep_save(value, connection=self.connection)
        return value